The proposed and ongoing research program is seeking to define the endocrine role of the thymus gland in the development and expression of host immunological resistance to the growth of neoplastic cells, which develop either spontaneously or after a viral challenge. These studies are being carried out in mice with a high incidence of spontaneous leukemia (AKR/J) and in mice (CBA/J) challenged with the Moloney sarcoma virus, a neoplastic tumor (Rhabdomyosarcoma)-inducing virus. The impetus for the present study is based upon the observation that a relatively crude thymus extract (thymosin, fraction 3) when administered before tumor induction enhances the resistance to progressive tumor growth induced by the Moloney sarcoma virus in both newborn and adult (immunosuppressed) mice. We have most recently succeeded in isolating thymosin in pure form. Purified thymosin will be used in the present study. Experimental approaches will include 1) suppression of host resistance (by neonatal thymectomy, adult thymectomy, whole body X-irradiation, cyclophosphamide, glucocorticoids, anti-lymphocyte serum, anti-thymosin serum, etc.), 2) enhancement of host resistance by thymosin and/or various populations of lymphoid cells and adjuvants, 3) evaluation of homing properties of normal and neoplastic lymphoid cells isotopically labeled with chromium-51, 4) analysis of changes in cell surface antigens of leukemic cells, e.g., theta antigen by the use of antisera, 5) responsiveness of lymphoid cells from tumor-bearing mice to mitogenic compounds, e.g., phytohemagglutinin, Fl cells (mixed leukocyte interaction) and 6) rosette-forming capacity of lymphoid cell populations before, during and after tumor induction.